Ion vacuum pumps



y 16, 1967 R. 1.. JEFSEN 3,319,875

ION VACUUM PUMPS Filed March 22, 1965 2 Sheets-$heet l FlG.-2

INVENTOR. ROBERT L. JEPSEN BY M 7776a,.

TTORNEY R. L. JEPSEN ION VACUUM PUMPS May 16, 1967 2 Sheets-Sheet 2 Filed March 22, 1965 FIG.5B

INVENTOR. ROBERT L. JEPSEN United States Patent 3,319,875 ION VACUUM PUD/[PS Robert L. Jepsen, Los Altos, Califi, assignor to Varian Associates, Palo Alto, Calif, a corporation of California Filed Mar. 22, 1965, Ser. No. 441,502 6 Claims. (Cl. 230-69) This invention relates to improved electrode configurations for ionic vacuum pumps to increase pumping Cfl'lciency generally, and in particular for enhancing the pumping of heavy noble gases such as argon, neon and krypton.

In my US. Patent No. 2,993,638, issued July 25, 1961, it is taught that the pumping performance of an ionic vacuum pump is enhanced by the provision of a sputter cathode grid, This grid is typically formed, for example, by a Wire mesh, or by a plurality of closely-spaced parallel, rectangular slats, supported at its margins in spaced relation from the pump envelope and with a portion of the slats disposed at glancing angles of incidence to the impinging ions.

When an ion having a certain kinetic energy collides with a surface of a sl-at at an oblique or glancing incidence, more cathode material will be sputtered than if the ion collides at normal incidence. In addition, there is a net build-up of sputtered cathode material on the back surface of the slat adjoining the incident surface. These net build-up areas are also subject to ion bombardment whereby the bombarding ions may be buried or covered over with subsequently sputtered material. This ion burial and covering over mechanism is especially beneficial when pumping the heavy noble gases such as argon, neon, krypton, and the like. Moreover, because of its increased noble gas pumping speed, argon instability, when pumping against an air leak, is eliminated.

An improved sputter cathode grid configuration forming the subject matter of my US. Patent No. 3,070,719, issued Dec. 25, 1962, includes the provision of supporting the wire mesh or cathode slats from a base member joined along the bottom edges of the grid. This base member increases thermal conductivity from the slats, mesh, or the like, forming the grid, assures mechanical rigidity and facilitates fabrication. The sputter cathode grid may be formed by slotting a cathode plate with a milling machine thereby forming on a base member a plurality of closely-spaced, parallel-directed slats extending outwardly therefrom, and either perpendicular to or slanted at an angle to the perpendicular to the base member. Alternatively, the cathode slats may be formed by milling a spiral slot in the cathode plate thereby providing an outwardly spiraling slot on the base member. In still other embodiments a plurality of sepaarte slats are disposed opposite and coaxial with the axes of individual anode cells, the slats being formed of cylindrical ring members of increasing diameter perpendicular to a base member, or of outwardly slanted frustoconical slats or ring segments of increasing diameter having parallel surface portions.

In a still further improved cathode grid configuration forming the subject matter of US. Patent No. 3,091,717, issued May 28, 1963, and assigned to the same assignee as the present invention, the portions of the sputter cathode grid structure disposed in alignment or registry with the axes of individual anode cells at the regions of maximum cathode erosion are made of increased density as compared to less intensely bombarded portions of the sputter cathode grid thereby lengthening the cathode operating life.

Simple linear slotting gives rise to a pumping speed for argon which is approximately 6-7 percent of that for nitrogen. This is sufiiicent to prevent argon instability when pumping against an air leak, but instability may still 0c- 3,3 N515 Patented May 16, 1967 cur when pumping against an argon leak. It has been recognized in the past that a more effective cathode configuration than that of linear slotting would be cylindrical ring members of increasing diameter disposed opposite and coaxial with the axes of individual anode cells, or preferably outwardly slanted frustoconical slats or ring segments of increasing diameter having parallel surface portions.

Briefly stated, in accordance with one teaching of the present invention there is provided an improved sputter cathode grid having greatly enhanced pumping speed for the noble gases comprising a plurality of spaced-apart, frustoconical slats or ring segments of increasing diameter disposed opposite and coaxial with the axes of individual anode cells, each having a surface subject to relatively intense ion bombardment, the angle which the surface of a respective one of said slats makes with an individual axis increasing with the radius of said slat.

It is known that the so called triode type of sputterion pump has a high pumping speed for argon even when the envelope is operated at anode potential. This result led to the speculation that a significant fraction of the noble gas ions are converted into energetic electron volts and higher) neutral atoms upon striking the surface of a sputter cathode. This effect is much larger for near grazing incidence than for normal incidence, Another source of energetic neutrals may be knock-on sputtering from the sputter cathodes of noble gas atoms previously buried there. Again, the magnitude of this effect Will be much greater for near grazing incidence.

With this further insight into the mechanisms of pumping noble gases one is motivated to design alternative pump structures which will take advantage of this insight. In particular, the cathode surfaces should be arranged such that: (1) ions strike one set of surfaces at near grazing incidence; and (2) energetic neutrals and sputtered cathode material are collected on a second set of surfaces; this second set of surfaces should be configured to receive only a small fraction, if any, of the ions directly.

In order to design optimized cathodes using this philosophy, one would have to know the angle of incidence of ions striking the cathodes as a function of radial position relative to the anode cell axis. It is known that most of the ions which strike the cathodes near the anode cell axis do so at normal incidence with only a few degrees of angular spread. It also appears that the average angle of incidence decreases monotonically with radius and that spread in angle increases with radius.

To maximize sputtering and the production of energetic neutrals, therefore, a plurality of spaced-apart frustoconical slats of increasing radius are disposed opposite and coaxial with the axes of individual cells each having a surface subject to relatively intense ion bombardment, the angle which the surface of a respective one of said slats makes with an individual axis increasing with the radius of said slot whereby each of these surfaces is disposed so as to approach near grazing incidence to the ion trajectories.

It is the object of the present invention to provide a magnetically confined glow discharge apparatus having an improved sputter cathode grid configuration to increase pumping efficiency generally and in particular for enhancig the pumping of heavy noble gases such as argon, neon and krypton.

One feature of the present invention is the provision in a magnetically confined flow discharge apapratus of a sputter cathode grid including a plurality of spaced apart frustoconical elements of increasing radius concentrically disposed about an axis, each having a surface portion exposed to relatively intense ion bombardment, the angle which the surface portion of a respective element makes with the axis increasing with increase in radius.

These and other features and advantages of the present invention will become apparent upon a perusal of the specification taken in connection with the accompanying drawing wherein;

FIG. 1 is a schematic diagram depicting a typical evacuation system utilizing a vacuum pump employing the novel features of the present invention;

FIG. 2 is a plan view partly in cross-section of a vacuum pump apparatus employing an improved sputter cathode grid of the present invention;

FIG. 3 is a cross-sectional view of FIG. 2 taken along the line 33 in the direction of the arrows;

FIG. 4 is a graph of average angle of incidence of ions impinging on a planar cathode surface vs. anode cell radius;

FIG. 5a is an enlarged fragmentary view of an individual anode cell and a portion of a cathode utilizing the novel sputter cathode grid of the present invention; and,

FIG. 5b is an enlarged fragmentary view of an individual anode cell and a portion of a cathode utilizing an alternate embodiment of the novel sputter cathode grid of the present invention.

Referring now to FIG. 1 there is shown in schematic block diagram form the improved magnetically confined glow discharge vacuum pump apparatus of the present invention as utilized for evacuating a given structure. More specifically, the vacuum pump 11 is connected by means of a conduit and through a first high vacuum joint 12 containing a pair of ultra-high vacuum flanges and a metal gasket (not shown) to a structure 13 which it is desired to evacuate. A vacuum sorption pump 14 is also connected to the structure 13 to be evacuated by means of conduits and through a second high vacuum joint 15 and a high vacuum valve 16. Frequently, a mechanical pump is used instead of the vacuum sorption pump 14. To evacuate the structure, the vacuum sorption pump 14 is immersed in a refrigerating liquid 17, as, for example, liquid nitrogen, held in an open vessel 18. Gas molecules are sorbed by the pump 14 from the structure 13 serving to reduce the pressure within the structure 13 to between 5 and or lower microns at which point the valve 16 is closed and the vacuum pump 11 started.

The pump 11 is supplied with operating potentials from a source 19 as, for example, a 60-cycle power line via a transformer 20. The secondary of the transformer 20 is provided with a rectifier 21 and a shunting smoothing capacitor 22 whereby a DC. potential may be applied between anode and cathode members of the vacuum pump 11, which pump will be more fully described below.

Referring now to FIGS. 2 and 3, a shallow, rectangular, cup-shaped member 23 as, for example, stainless steel is closed off at its flanged open end by a rectangular closure plate 24 welded about its periphery to the flanged portion of the member 23, thereby forming a substantially rectangular vacuum-tight envelope 25.

A rectangular cellular anode 26 as of, for example, t-itanium is carried upon the end of a conductive rod 27 as of, for example, stainless steel which extends outwardly of the rectangular vacuum envelope through an aperture in a short sidewall thereof. The conductive rod 27 is insulated from and carried by the vacuum envelope 25 through the intermediary of annular insulator frames 28, 29, 30 as of, for example, Kovar and cylindrical insulator 31 as of, for example, alumina ceramic. The free end of the rod 27 serves to provide a terminal for providing a positive anode voltage with respect to two substantially rectangular slatted cathode plates 32. The cathode plates 32 are made of a reactive metal and are mechanically locked into position against the large fiat sidewalls of the vacuum envelope 25 via the intermediary of two cathode spacer plates 33.

The cathode spacer plates 33, as of stainless steel, are provided with semi-cylindrical cars 34 struck therefrom for assuring proper spacing between the cathode plates 32. In a preferred embodiment, the anode-to-cathode spacing lies within the range of between /a and inch. The cathode plates 32 may be made of any one of a number of reactive cathode metals such as, for example, titanium, chromium, zirconium, gadolinium and iron.

Another sidewall 35 of the vacuum envelope 25 is apertured to receive the hollow conduit, which may be of any convenient inside diameter commensurate with the desired pumping speed. The hollow conduit communicates with the structure 13 which is desired to be evacuated and is provided with a suitable mounting flange.

A circular radial shield 36 as of, for example, molybdenum is carried transversely of the conductive rod and is disposed inside the first frame member 28 for shielding the insulator 31 from sputtered cathode material which might otherwise coat the insulator 31 and produce un wanted voltage breakdown or current leakage thereacross. An annular spring 37 is positioned circumscribing the frame member 29 to provide a quick disconnection between the power connector, not shown, and the pump 11.

A horsehoe-shaped permanent magnet 38 is positioned with respect to the rectangular vacuum envelope 25 such that the magnetic field threads through the individual cellular elements of the anode 26. Although permanent magnets are shown, electromagnets may be used advantageously.

In operation, a positive potential of 2.0 kilovolts or more is applied to the anode 26 via the conductive rod 27. The vacuum envelope 25 and, therefore, the cathode plates 32 are preferably operated at ground potential to reduce hazard to operating personnel. With these potentials applied, a region of intense electric field is produced between the cellular anode 26 and cathode plates 32. This electric field produces a breakdown of gas within the pump resulting in a glow discharge within the cellular anode 26 and between the anode 26 and the cathode plates 32. The glow discharge results in positive ions being driven into the cathode plates 32 to produce dislodgment of reactive cathode material which is thereby sputtered onto the nearby anode 26 to produce gettering of molecules in the gaseous stage coming in contact therewith. In this manner the pressure within the vacuum envelope 25 and, therefore, the structure 13 communicating therewith are evacuated.

Noble gases, such as argon and helium, because of their lack of chemical activity are pumped primarily by ion burial in the cathode plates 32. In the absence of subsequent diffusion into the cathode interior, pumping by this mechanism should cease when the rate of sputter ing out of previously buried gas atoms becomes equal to the rate at which ions are being buried. However, continued pumping of the noble gases does occur in regions of certain cathode configurations where there is ion burial in regions of net buildup of sputtered cathode material. Therefore, it has been found that the prior art sputter cathode grid, which is characterized by increased sputtering and regions where there is both a net buildup of sputtered cathode material and ion burial, provides the needed increased pumping speed for noble gases, over that obtained by a flat cathode.

Referring now to FIG. 4, it is known that most ions which strike the cathode near the anode cell axis (r=0) do so at normal incidence (0=1r/2) and it would appear that the average angle of incidence decreases monotonically with radius. With this in mind, a geometry was sought so that ions would strike one set of surfaces at near grazing incidence and energetic neutrals and sputtered cathode material would be collected on a second set of surfaces, which surfaces would receive only a small fraction of ions directly.

Referring now to FIG. 5, there are shown preferred embodiments of the present invention wherein the face of the cathode plate 32, which is disposed adjacent the open ends of the cells of the anode 26, is provided with a plurality of spaced-apart frustoconical slats 39 of increasing radius, each having a surface subject to relatively intense ion bombardment. In FIG. 5a the front surface 40 of the slats 39 are subject to relatively intense ion bombardment While in FIG. 5b the back surfaces 41 are subject to the relatively intense ion bombardment. The angle which the bombarded surface of a respective one of the slats makes with the axis A of the cell illustrated increases with the radius of the slat so that each of these surfaces is disposed so as to approach grazing incidence to the ion trajectories. Stated in another way, the angle a which the bombarded surface of an individual slat makes with the cathode plate 32 disposed perpendicularly to the axis A decreases with increase in radius of the .slat (a ct a The slats 39 in FIG. 5a are arranged so that the bombarded surface 40 is at a slightly smaller angle than the average angle of incidence of ions at that point whereby relatively intense ion bombardment occurs on the surface 40 resulting in a net buildup of material within the bottom of the slots and on the back surface 41 of the slat 39 adjoining the incident surface 40. This may generally be referred to as outside sputtering. On the other hand, in FIG. 5b, the slats 39 are arranged so that the bombarded surface 41 is at a slightly larger angle than the average angle of incidence of ions at that point whereby relatively intense ion bombardment occurs on the surface 41 resulting in a net buildup of material within the bottom of the slot and on the front surface 40 of the slat 39 adjoining the incident surface 41. This may generally be referred to as inside sputtering.

The distance between adjacent slats should be small in comparison with the diameter of an anode cell and the width of the slat should be smaller than the distance between slats. The depth of the slot should be larger than its width. The cathode slats preferably are brazed to the cathode plates at their contacting side edges In FIG. 5a a frustoconical plug 42 is provided on the cathode plate in substantial registry with the axis A of the individual cell of anode 26. The outer slatted surfaces 43 of the plug are slanted at grazing incidence to the intense ion bombardment thereon. In FIG. 5b the plug 44 is simply cylindrical, the outer surface 45 being a region of net buildup as a result of the inside sputtering from the back surface 41 of the adjoining slats 39.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A sputter cathode grid for a magnetically confined glow discharge apparatus comprising: means forming a cold cathode base plate; and means forming a cold cathode grid extending outwardly from said cathode base plate, said grid including a plurality of spaced-apart, concentrically disposed frustoconical elements of increasing radius each having a surface portion exposed to relatively intense ion bombardment, the angle which the surface portion of a respective element makes with said base plate decreasing with increase in radius of said element. 2. A sputter cathode grid for a magnetically confined glow discharge apparatus comprising: means forming a cold cathode base plate; and means forming a cold cathode grid extending outwardly from said cathode base plate, said grid including a plurality of spaced-apart, con centrically disposed frustoconical elements of increasing radius each having a front surface subject to relatively intense ion bombardment and having a back surface subject to less intense ion bombardment, the angle which the front surface of a respective element makes with said base plate decreasing With increase in radius of said element.

3. A grid according to claim 2 including a relatively wide frustoconical plug extending outwardly from said cathode base plate and being centrally disposed with respect to said frustoconical elements, said plug having a surface subject to relatively intense ion bombardment, the angle which the surface of said plug makes with said base plate being smaller than the angle Which the front surface of the adjoining frustoconical element makes with said base plate.

4. A sputter cathode grid for a magnetically confined glow discharge apparatus comprising: means forming a cold cathode base plate; and means forming a cold cathode grid extending outwardly from said cathode base plate, said grid including a plurality of spaced-apart concentrically disposed frustoconical elements of increasing radius each having a back surface subject to relatively intense ion bombardment and having a front surface subject to less intense ion bombardment, the angle which the back surface of a respective element makes with said base plate decreasing with increase in radius of said element.

5. The grid according to claim 4 having a relatively wide cylindrical plug centrally disposed with respect to said frustoconical elements.

6. A magnetically confined glow discharge sputter-ion vacuum pump apparatus including: an evacuable envelope; an apertured anode electrode disposed within said envelope and defining a path for a glow discharge through the aperture in said anode electrode; a sputter cathode grid disposed within said envelope opposite said anode electrode for ion bombardment by ions produced by the glow discharge in use, said grid including a first and second slat symmetrically, outwardly spaced from the axis of said apertured anode electrode, said second slat being outwardly spaced of said first slat, each of said slats having a surface subject to relatively intense ion bombardment, the angle which the surface of said first slat makes with said anode axis being less than the angle which the surface of said second slat makes with said anode axis; means for producing and directing a magnetic field axially through said anode electrode, and means for applying a potential to said anode electrode positive with respect to said cathode grid.

References Cited by the Examiner UNITED STATES PATENTS 3,141,986 7/1964 Lloyd 313-7 DONLEY J. STOCKING, Primary Examiner. LAURENCE V. EFNER, Examiner. 

1. A SPUTTER CATHODE GRID FOR A MAGNETICALLY CONFINED GLOW DISCHARGE APPARATUS COMPRISING: MEANS FORMING A COLD CATHODE BASE PLATE; AND MEANS FORMING A COLD CATHODE GRID EXTENDING OUTWARDLY FROM SAID CATHODE BASE PLATE, SAID GRID INCLUDING A PLURALITY OF SPACED-APART, CONCENTRICALLY DISPOSED FRUSTOCONICAL ELEMENTS OF INCREASING RADIUS EACH HAVING A SURFACE PORTION EXPOSED TO RELATIVELY INTENSE ION BOMBARDMENT, THE ANGLE WHICH THE SURFACE PORTION OF A RESPECTIVE ELEMENT MAKES WITH SAID BASE PLATE DECREASING WITH INCREASE IN RADIUS OF SAID ELEMENT. 